The present invention relates to a method of accessing a communication medium.
Communication traffic between communication apparatus, such as a base station and a plurality of communication stations forming a Local Area Network (LAN), can comprise asynchronous traffic or isochronous traffic.
Asynchronous traffic exhibits unpredictable traffic timing, at which a fast burst of packets is generated, and while such traffic requires fast response times in the stations, a wide range of instantaneous packet delays can be tolerated provided the average delay in the stations is within acceptable limits. An example of asynchronous traffic communication is the communication that occurs between computer systems when a file has to be transferred relatively infrequently but at a high speed once the communication commences. A large LAN bandwidth must be available to the asynchronous traffic to allow for the fast transfer of the file.
Communication-medium access protocols that are established having regard to the above mentioned characteristics of asynchronous stations are based on a so-called Distributed Access Protocol in which the medium access procedure is shared by all the stations connected to the medium. An example of a Distributed Access Protocol is the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol used as part of the Ethernet and IEEE 802.3 communication protocols.
Isochronous traffic requires a predictable channel bandwidth and can tolerate a long average packet-delay in the stations provided the variations in the delays of different packets are within narrow limits. Also, isochronous packets that are not delivered within a specified delay are generally discarded. The quality of the isochronous communication depends upon the application requirements, but generally a pre-defined bandwidth is required along with a bit-error-rate limit or packet-error-rate limit. An example of isochronous traffic is so-called packetized-voice traffic. This traffic comprises voice data in the form of a steady stream of packets that needs a limited bandwidth and also a regular delay characteristic to achieve a steady packet stream for the relatively long call duration. Other examples are industrial processes that need to react to known situations or events at an accurately determinable time. Networks that can handle isochronous traffic employ so-called Connection Oriented access protocols in which a connection between the stations is established and the stations are assigned a channel with a well defined bandwidth and delay characteristic. An example of such a network is the Integrated Services Digital Network (ISDN) which employs a Time Division Multiple Access (TDMA) protocol to service multiple stations in the network.
From the above it can be seen that asynchronous and isochronous communication each have very different requirements and characteristics. Thus, when asynchronous and isochronous communication is required in the same LAN, the asynchronous packets are transferred via a different medium (i.e. cable) from the isochronous packets and different access protocols are employed on the different media. This disadvantageously increases the cost and complexity of the LAN and is particularly disadvantageously restrictive in that isochronous and asynchronous traffic cannot be readily provided in a wireless LAN where the stations have to share the same wireless medium. Such wireless LANs are being increasingly provided for devices such as notebook P.C.s or Personal Digital Assistance (PDA) devices. It would be advantageous if such devices could provide for the communication of real-time mobile packets along with data packets, and so a method of accessing a communication medium is required that provides for the sharing of the same medium by asynchronous and isochronous traffic.